To be honest I recently made a character after my username. I didn't think much of it when I saw the others but I decided to give it a go. Now I have 2 Hakumi ( a feline & canine version ) and a Haku ( feline ), Lol.
My main character is named Koko, which is like my nickname. {My username on most things is Koeya,
but I've been nicknamed "Koko" for at least... 3-4 years, I believe?} So in a way, I guess that is an extension of my username.
I think it's just easier for people to remember, & when people refer to me, whether it be by character name or username, they'll likely still know who I am, because of the overlap. :)
Most of my characters were made on Dragon's Den, aha. They were, Naruto, And Kyuubi.